Circuit for operating an electric motor

Abstract

A circuit for operating an electric motor ( 11 ) includes a monitoring arrangement ( 24 ) for ascertaining an increased load state of the electric motor ( 11 ). The monitoring arrangement ( 24 ) compares an rpm difference (N D ) between a predetermined nominal rpm value (N SOLL ) and an actual rpm value signal (N IST ) with a predetermined limit value. If the limit value is exceeded, the monitoring arrangement ( 24 ) emits an overload signal ( 25 ) that reduces the nominal rpm value (N SOLL ) or a control variable (S N ) supplied to a drive stage ( 18 ), or drops it to zero. The circuit is particularly suited for use with a fan, in which instance the ventilator ( 22 ) cools the electric motor ( 11 ) and/or the drive stage ( 18 ).

What is claimed is:  
     
       1. An improved circuit for operating a DC electric motor, the circuit having an arrangement that produces a signal serving as a measure for the rpm of the electric motor and having a monitoring arrangement for detecting an increased load state of the electric motor, wherein the improvement comprises: 
       the monitoring arrangement ( 24 ) compares an rpm difference (N D ) between a predetermined nominal rpm value (N SOLL ) and the actual rpm determined from the actual rpm value signal (N IST ) to a predetermined limit value, and emits an overload signal ( 25 ) if the limit value is exceeded.  
     
     
       2. A circuit according to claim  1 , wherein the limit value is a function of the predetermined nominal rpm value (N SOLL ). 
     
     
       3. A circuit according to claim  1 , wherein the monitoring arrangement ( 24 ) includes a first timer ( 26 ) that takes into account a run-up process of the electric motor ( 11 ) with respect to setting of the nominal rpm value (N SOLL ) by suppressing the overload signal ( 25 ). 
     
     
       4. A circuit according to claim  1 , wherein the arrangement that produces a signal serving as a measure for the rpm includes an rpm sensor. 
     
     
       5. A circuit according to claim  1 , wherein the arrangement that produces a signal serving as a measure for the rpm detects the motor current and the motor terminal voltage, and determines the rpm from the detected motor current and the motor terminal voltage. 
     
     
       6. A circuit according to claim  1 , in combination with a fan that is driven by the electric motor ( 11 ), wherein the circuit further comprises a drive stage ( 18 ) for driving the electric motor ( 11 ) and the fan cools at least one of the electric motor ( 11 ) and the drive stage ( 18 ) using air currents. 
     
     
       7. An improved circuit for operating a DC electric motor, the circuit having an arrangement that produces a signal serving as a measure for the rpm of the electric motor and having a monitoring arrangement for detecting an increased load state of the electric motor, wherein the improvement comprises: 
       the monitoring arrangement ( 24 ) compares an rpm difference (N D ) between a predetermined nominal rpm value (N SOLL ) and the actual rpm determined from the actual rpm value signal (N IST ) to a predetermined limit value, and emits an overload signal ( 25 ) if the limit value is exceeded; and  
       the current further comprises a nominal value reducer ( 13 ) that receives the overload signal ( 25 ) and that lowers the nominal rpm value (N SOLL ) to a limited nominal rpm value (N′ SOLL ) in response to the overload signal ( 25 ).  
     
     
       8. An improved circuit for operating a DC electric motor, the circuit having an arrangement that produces a signal serving as a measure for the rpm of the electric motor and having a drive stage for driving the electric motor in response to a control variable, the circuit additionally having a monitoring arrangement for detecting an increased load state of the electric motor, wherein the improvement comprises: 
       the monitoring arrangement ( 24 ) compares an rpm difference (N D ) between a predetermined nominal rpm value (N SOLL ) and the actual rpm determined from the actual rpm value signal (N IST ) to a predetermined limit value, and emits an overload signal ( 25 ) if the limit value is exceeded; and  
       the circuit further comprises a regulator ( 16 ) which generates the control variable (S N ) as a function of the rpm difference (N D ), the regulator ( 16 ) including a control variable limiter ( 17 ) that receives the overload signal ( 25 ) and that reduces the control variable (S N ) supplied to the drive stage ( 18 ) to a predetermined value in response to the overload signal ( 25 ).  
     
     
       9. An improved circuit for operating an electric motor, the circuit having an arrangement that produces a signal serving as a measure for the rpm of the electric motor and having a monitoring arrangement for detecting an increased load state of the electric motor, wherein the improvement comprises: 
       the monitoring arrangement ( 24 ) compares an rpm difference (N D ) between a predetermined nominal rpm value (N SOLL ) and the actual rpm determined from the actual rpm value signal (N IST ) to a predetermined limit value, and emits an overload signal ( 25 ) if the limit value is exceeded, and  
       the monitoring arrangement ( 24 ) includes a timer ( 27 ) that limits the duration of the overload signal ( 25 ) to a time (T aus ) predetermined by the timer ( 27 ), and a further timer ( 28 ) that predetermines a time (T ein ) during which the overload signal ( 25 ) is then suppressed.  
     
     
       10. A circuit according to claim  9 , further comprising a drive stage ( 18 ) for driving the electric motor ( 11 ), and wherein the two times (T aus , T ein ) predetermined by the two timers ( 27 ,  28 ) are measured so as to preclude thermal overload of at least one of the electric motor ( 11 ) and the drive stage ( 18 ). 
     
     
       11. A circuit for operating a motor in response to a nominal rpm signal, comprising: 
       means for producing an actual rpm signal which serves as a measure of the rpm of the motor;  
       means for generating a control variable signal which is a function of the difference between the nominal and actual rpm signals;  
       means for comparing the difference between the nominal and actual rpm signals to a limit value and generating an overload signal if the difference exceeds the limit value;  
       means for reducing the control variable signal if the overload signal appears; and  
       means for driving the motor in response to the control variable signal.  
     
     
       12. A circuit according to claim  11 , wherein the limit value is a function of the nominal rpm signal. 
     
     
       13. A circuit according to claim  11 , wherein the means for generating a control variable signal comprises an adder which receives the nominal and actual rpm signals and generates a different signal from them, and a nominal value reducer which reduces the nominal rpm signal before it is received by the adder if the overload signal appears. 
     
     
       14. A circuit according to claim  11 , wherein the means for generating a control variable signal comprises an adder which receives the nominal and actual rpm signals and generates a difference signal from then, and a regulator which generates the control variable signal from the difference signal, the regulator including a control variable limiter which limits the magnitude of the control variable signal if the overload signal appears. 
     
     
       15. A circuit according to claim  11 , wherein the means for generating an overload signal comprises a timer which suppresses the overload signal during run-up of the motor. 
     
     
       16. A circuit according to claim  15 , wherein the means for generating an overload signal further comprises another timer which limits the duration of the overload signal and an additional timer which then times an interval during which the overload signal is suppressed. 
     
     
       17. A circuit according to claim  11 , wherein the motor is a DC motor, wherein the control variable signal is a pulse-width modulated signal, and wherein the means for driving the motor selectively connects the motor to a power source in response to the control variable speed. 
     
     
       18. A circuit according to claim  11 , wherein the motor is a DC motor.